1. Field of the Invention
The present invention relates to a developing apparatus which is applicable to an image forming apparatus such as an electrophotographic apparatus.
2. Related Background Art
FIG. 9 shows an example of a conventional image forming apparatus having a developing apparatus. FIG. 9 is a vertical cross-sectional view showing the schematic structure of the image forming apparatus, and referring to the figure, the outline of the conventional image forming apparatus will be described.
A photosensitive drum (image bearing member) 111 is rotationally driven in a direction indicated by an arrow RI so that after a surface of the photosensitive drum 111 is uniformly charged by a charging roller (charging device) 103, it is subject to exposure by an exposing device 102 to thereby form an electrostatic latent image on the surface of the photosensitive drum 111. The electrostatic latent image is developed as a toner image by a developing apparatus 208. The toner image on the photosensitive drum 111 is transferred to a transfer material 114 by a transfer roller (transfer device) 110. The transfer material 114 which has been received in a sheet feed cassette 117 is supplied by a sheet feed roller 116 and so on. Toner which is not transferred and remains on the surface of the photosensitive drum 111 from which the toner image has been transferred is removed by a cleaning blade 113 of a cleaning device 112 for succeeding image formation. On the other hand, the toner image which has been transferred to the transfer material 114 is fixed onto the surface of the transfer material 114 by a fixing device 115 and thereafter discharged to the exterior of an image forming apparatus main body 101. With the above operation, image formation is completed.
FIG. 10 shows an example of a conventional developing apparatus. FIG. 10 is an enlarged diagram of the developing apparatus 208 shown in FIG. 9. Referring to the figure, the outline of the conventional developing apparatus will be described.
The developing apparatus 208 shown in the figure is comprised of a developing apparatus using one-magnetic-component toner. A developing sleeve (developer bearing member) 105 is comprised of a non-magnetic sleeve which is formed of a pipe made of aluminum or stainless steel and rotatably supported in a direction indicated by an arrow R2. Inside of the developing sleeve 105 is fixedly disposed a magnet 106 having a plurality of N-poles and S-poles which are alternately formed. A surface of the developing sleeve 105 is so processed as to provide an appropriate surface roughness so that a desired amount of toner can be carried. The surface of the developing sleeve 105 abuts against one end of an elastic blade (developer regulation member) 107a under a given pressure. The elastic blade 107a is made of, for example, urethane rubber, silicon rubber or the like and the end of the elastic blade 107a is fixed to a support plate 107b. Toner 109 which has been attracted to the surface of the developing sleeve 105 due to a magnetic force of the magnet 106 is supplied with an appropriate amount of charges by a frictional charge caused by bearing and carrying the toner 109 on the developing sleeve 105 and a frictional charge caused by sliding friction between the developing sleeve 105 and the elastic blade 107a at the time where the toner 109 is regulated to an appropriate amount by the elastic blade 107a. The toner 109 is then carried to a developing region.
Also, the developing apparatus 208 is designed in such a manner that, as shown in FIG. 15, rollers 209 are disposed at both end portions of the developing sleeve 105, and the rollers 209 are allowed to abut against the photosensitive drum 111 so that a given interval is provided between the surface of the developing sleeve 105 and the surface of the photosensitive drum 111. A developing sleeve gear 212 is fixed onto one end of the developing sleeve 105 so that the developing sleeve 105 is rotationally driven by a drive force which is transmitted to the developing sleeve gear 212 from a photosensitive drum gear 211 integral with the photosensitive drum 111.
As means for detecting the remaining amount of toner in the developing apparatus 208 thus structured, there has been known, up to now, a technique in which a variation in current which is induced by an a.c. developing bias supply power source 118 is detected by an antenna member 108 disposed in parallel with the developing sleeve 105, using a variation in impedance which is accompanied by a variation in the amount of toner between the developing sleeve and the antenna as shown in FIG. 10, to thereby estimate the remaining amount of toner (hereinafter referred to as "induced current detecting means). The power source for supplying the developing bias includes not only the above-described a.c. developing bias supply power source 118 but also a d.c. developing bias supply power source 119, by which a developing bias where a.c. voltage and d.c. voltage are superposed on each other is supplied to the above-described developing sleeve 105.
The induced current detecting means is made up of the respective members 120 to 123 in FIG. 10. Reference numeral 120 denotes a capacitor having an electrostatic capacitance equal to that in a state where no toner exists. The electrostatic capacitance of the capacitor 120 and an electrostatic capacitance detected by the antenna member 108 are compared with each other after they pass through diodes 121 and 122, respectively, to thereby judge whether the toner exists or not.
In the case of detecting the remaining amount of toner, the antenna member 108 disposed inside of a developing container 104 in the longitudinal direction may prevent the movement of toner. In particular, because a toner wall is liable to be formed between the lower portion of the developing container 104 and the antenna member 108, a method has been conventionally applied in which an agitating member 130 is disposed at the lower portion of the antenna member 108 so that the supply of toner to the developing sleeve 105 is smoothed while toner is being unraveled.
As the agitating member 130, there is used a crank-shaped bar member as shown in FIG. 11, which is rotated in a direction indicated by an arrow R3 with both end portions thereof as a rotating center. This is a conventional method.
Also, an agitating member 7 is so disposed as to unravel the toner within the developing container 104 and deliver the toner to the agitating member 130. The agitating member 7 is formed of a crank-shaped bar member as in the agitating member 130 as shown in FIG. 12, which is rotated in a direction indicated by an arrow R4 in FIG. 10 with both end portions thereof as a rotating center. In general, a driving force that permits the agitating member 130 and the agitating member 7 to rotate is obtained, for example, by lowering the driving force transmitted from the above-described developing sleeve gear 212 (refer to FIG. 15) to an appropriate rotating speed through a gear train.
However, for example, in the image forming apparatus of the digital system which performs exposure by a laser beam, with the decreased diameter of the toner for the purpose of improving the reproducibility of one dot, a tendency is given to lower the density of an image for some time after the developing apparatus 208 structured in the above manner starts to operate.
FIG. 13 is a graph representative of an initial image density transition with respect to a difference in toner average (center) particle diameters. Although the initial density is low in both of cases where the average particle diameters are 6 and 8 .mu.m, the case where the average particle diameter is smaller, that is, 6 .mu.m is more remarkable for the initial lower density. The level of the initial low density in the case where the average particle diameter is 8 .mu.m or more is 1.4 or more without any problems. On the other hand, in the case where the toner less than 8 .mu.m in the average particle diameter is employed, it is found that the level of the initial low density is lowered down to a level which should be desirably improved.
According to the present inventors' study, it has been proved that the cause to lower the density is because there is a correlation between the amount of fine particles relatively small in particle diameter among the toner and the initial density of solid image (all black) (Macbeth reflection densitometer made by Macbeth Co., was used for density measurement) as shown in FIG. 16, and the density is lowered more as the amount of fine particles increases. The fine particles of toner are considerably different in triboelectricity from the normal toner. It has been proved that, in an initial stage, a tendency is given to allow the particles relatively small in the toner within the developing apparatus 208 to collect in the vicinity of the developing sleeve 105 at a stage where the developing apparatus starts to operate, with the result that the triboelectricity distribution of the toner coated on the developing sleeve 105 is broadened (the ratio of toner having an optimum triboelectricity for development to all the toner is reduced), and developing capacity is lowered. Because the existence of the toner small in particle diameter leads to any problem, the above phenomenon is liable to remarkably appear as the average particle diameter of toner is smaller. In addition, it has been found that this phenomenon becomes more remarkable as a pattern small in toner consumption continues to be printed (for example, solid image immediately after blank copy (all white) continues to be printed is more lowered). This is because the amount of fine particles in the vicinity of the developing sleeve 105 increases more as the toner consumption is reduced.
FIG. 14A is a diagram showing the movement of toner within the conventional developing apparatus 208 using the crank-shaped agitating member 130. In the figure, the supply of new toner from the side of the developing container 104 is conducted in two ways, that is, conducted by allowing the new toner to pass through a space below the agitating member 130 as indicated by an arrow 4 and conducted by allowing the new toner to flow in a space between the agitating member 130 and the antenna member 108 due to the self weight action of toner as indicated by an arrow 5 as shown in FIG. 14B. The toner that has flown in the vicinity of the developing sleeve 105 in the directions indicated by the arrows 4 and 5 is attracted by the magnetic force of the magnet 106 inside of the developing sleeve 105. Thereafter, the toner is coated on the developing sleeve 105 by means of the elastic blade 107. The toner which has not been coated on the developing sleeve 105 is circulated in a direction indicated by an arrow 2 or an arrow 3. This circulation of toner contributes to the supply of triboelectricity to toner. According to the present inventors' study, it has been found that the amount of toner which flows into a space between the agitating member 130 and the antenna member 108 when the space is broadened during the rotation of the agitating member 130 is far larger than the amount of toner which is pushed and supplied in a rotationally advancing direction with the rotation of the agitating member 130. It has been also found that the flow of the large amount of toner causes the initial low density to occur. In other words, it has been proved that in a structure having the antenna member 108 as in the above-described structure of the developing apparatus 208, there is required a structure in which the flow of toner into the space between the crank-shaped agitating member 130 and the antenna member 108 is so controlled as to optimize the supply of toner toward the side of the developing sleeve 105.
As means for eliminating the above problem, there has been first proposed that the particle diameter of toner is unified (the fine particle side is cut during manufacturing). However, because the yield at the time of manufacturing the toner is remarkably deteriorated, thereby leading to the high costs, such means cannot be realized.
Also, there has been proposed in Japanese Patent No. 2682003 that, at the side of the developing sleeve within a toner containing (receiving) tank in the developing apparatus, a partition member is disposed at an inner lower side of the developing apparatus, and a supply chamber for supplying the toner by the agitating member is disposed within a cell in the toner receiving tank which is disposed between the partition member and the developing sleeve, in such a manner that the toner within the cell is used for development in priority to stably supply the toner having a uniform particle diameter. However, because a toner carrying force sufficient to get over the wall from the lower side is demanded, the performance request to the agitating structure is severe, thereby leading to the increased costs. Also, in a system where the center of gravity of the developing container is located above the rotating center of the developing sleeve, the inflow of toner from the arrows 4 and 5 in FIGS. 14A and 14B can be suppressed. However, in the case where an agitating member small in rotational torque and low in the costs is employed as the agitating member, or in the case where the supply of toner is conducted due to only the self-weight drop of toner without the provision of the agitating member, the absolute supply amount of toner to the above-described cell is liable to be lacking and the supply of toner to the developing sleeve becomes unstable, resulting in the possibility that a blank stripe occurs on the image even if a large amount of toner exists in the developing container.
Further, there has been proposed in Japanese Patent Application Laid-open No. 10-104943 that a protective wall is projected downward from an upper wall portion of the developing chamber in the extreme vicinity of the toner layer regulation member to the same degree as the toner layer regulation member. However, even in this structure, the inflow of toner in a direction indicated by the arrow 5 shown in FIG. 14B cannot be suppressed, thereby making it possible to prevent the initial density from being lowered.